Digitally controlled oscillator having enhanced control resolution

Abstract

A signal generator circuit having enhanced control resolution includes a variable frequency oscillator having a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal. The second signal is afforded a weight which is substantially equal to a weight of a least significant bit of the first signal. The variable frequency oscillator is operative to generate an output signal having a frequency which varies as a function of the first and second signals. The signal generator circuit further includes a first counter, a first divider and a modulation circuit. The first counter is configurable for generating an output count based at least in part on a difference between a third signal presented to the signal generator circuit and a fourth signal indicative of the output signal, the output count including the first signal. The first divider has a selectable divide value associated therewith and is configurable for receiving the output signal and for generating the fourth signal based on the divide value. The modulation circuit is adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal.

1. A signal generator circuit having enhanced control resolution, comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal. 
 
   
   
     2. A signal generator circuit having enhanced control resolution, comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; 
 wherein the modulation circuit is configured such that a least significant bit of the output count generated by the first counter is modulated by a most significant bit of the fourth signal generated by the first divider. 
 
   
   
     3. The circuit of  claim 2 , wherein most significant bit of the output count is modulated by a least significant bit of the fourth signal. 
   
   
     4. A signal generator circuit having enhanced control resolution, comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; 
 wherein, for an n-bit output count supplied to the modulation circuit, the modulation circuit comprises n−1 multiplexers, each of the multiplexers including a first input adapted for receiving a corresponding bit of the n-bit output count, a second input adapted for receiving an output signal of a preceding one of the multiplexers, and a control input adapted for receiving a corresponding bit of the fourth signal, where n is an integer greater than one. 
 
   
   
     5. The circuit of  claim 4 , wherein the first input of a first of the multiplexers is adapted to receive a most significant bit of the output count, the first multiplexer being operative to generate the modulated bit signal, and the control input of the first multiplexer is adapted to receive a most significant bit of the fourth signal. 
   
   
     6. The circuit of  claim 1 , wherein the first counter is further configurable for generating a fifth control signal in response to the first counter generating one of an overflow and an underflow, the divide value in the first divider being selected based on the fifth signal. 
   
   
     7. A signal generator circuit having enhanced control resolution, comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; and 
 a second counter configurable for generating an output count in response to the first counter generating one of an overflow and an underflow, the output count of the second counter representing a duration of one of the overflow and the underflow of the first counter, the divide value of the first divider being selected based on the output count of the second counter. 
 
   
   
     8. A signal generator circuit having enhanced control resolution, comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals: 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; 
 wherein the difference signal being a first value causes the output count of the first counter to increase and the frequency of the output signal from the variable frequency generator to decrease, and the difference signal being a second value causes the output count of the first counter to decrease and the frequency of the output signal from the variable frequency generator to increase. 
 
   
   
     9. The circuit of  claim 1 , wherein the difference signal being a first value causes the output count of the first counter to increase and the frequency of the output signal from the variable frequency generator to increase, and the difference signal being a second value causes the output count of the first counter to decrease and the frequency of the output signal from the variable frequency generator to decrease. 
   
   
     10. A signal generator circuit having enhanced control resolution, comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; 
 wherein the variable frequency generator comprises a plurality of delay stages including first and last stages, the plurality of delay stages being connected in a ring configuration, such that an output of a given delay stage is coupled to an input of a successive delay stage, the output of the last stage being operatively coupled to the input of the first stage. 
 
   
   
     11. The circuit of  claim 1 , wherein the first counter comprises an up/down counter. 
   
   
     12. The circuit of  claim 1 , wherein the first divider comprises a programmable counter. 
   
   
     13. The circuit of  claim 1 , wherein the comparator comprises at least one of a phase detector and a frequency detector. 
   
   
     14. The circuit of  claim 1 , wherein the first signal comprises a digital control word and the second signal comprises a modulated bit signal. 
   
   
     15. The circuit of  claim 1 , wherein the variable frequency oscillator is a digitally controlled oscillator. 
   
   
     16. The circuit of  claim 1 , wherein the signal generator circuit is included in a digital phase locked loop circuit. 
   
   
     17. An integrated circuit including at least one signal generator circuit having enhanced control resolution, the at least one signal generator circuit comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal. 
 
   
   
     18. An integrated circuit including at least one signal generator circuit having enhanced control resolution, the at least one signal generator circuit comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; 
 wherein the modulation circuit is configured such that a least significant bit of the output count generated by the first counter is modulated by a most significant bit of the fourth signal generated by the first divider. 
 
   
   
     19. An integrated circuit including at least one signal generator circuit having enhanced control resolution, the at least one signal generator circuit comprising:
 a variable frequency oscillator including a first control input adapted to receive a first signal and at least a second control input adapted to receive a second signal, the second signal being afforded a weight which is substantially equal to a weight of a least significant bit of the first signal, the variable frequency oscillator being operative to generate an output signal having a frequency which varies as a function of the first and second signals; 
 a comparator configurable for receiving a third signal and a fourth signal, the third signal being an input signal presented to the signal generator circuit and the fourth signal being representative of the output signal from the variable frequency oscillator, the comparator generating a difference signal representative of a difference between at least one of a phase and a frequency of the third and fourth signals; 
 a first counter configurable for generating an output count based at least in part on the difference signal from the comparator, the output count being a digital representation of the difference signal, the output count comprising the first signal; 
 a first divider connected between the variable frequency oscillator and the comparator, the first divider having a selectable divide value associated therewith and being configurable for receiving the output signal from the variable frequency oscillator and for generating the fourth signal based on the divide value; and 
 a modulation circuit adapted to receive at least a portion of the output count and the fourth signal and to generate the second signal as a function of the output count and the fourth signal; 
 wherein, for an n-bit output count supplied to the modulation circuit, the modulation circuit comprises n−1 multiplexers, each of the multiplexers including a first input adapted for receiving a corresponding bit of the n-bit output count, a second input adapted for receiving an output signal of a preceding one of the multiplexers, and a control input adapted for receiving a corresponding bit of the fourth signal, where n is an integer greater than one. 
 
   
   
     20. The integrated circuit of  claim 19 , wherein the first input of a first of the multiplexers is adapted to receive a most significant bit of the output count, the first multiplexer being operative to generate the modulated bit signal, and the control input of the first multiplexer is adapted to receive a most significant bit of the fourth signal.